1. Field of the Invention
This invention relates in general to circuits and electronic devices, and more particularly, to circuits including switches for electronic devices and methods of using such electronic devices.
2. Description of the Related Art
Electronic devices, including organic electronic devices, continue to be more extensively used in everyday life. Examples of organic electronic devices include Organic Light-Emitting Diodes (“OLEDs”). Active Matrix OLED (“AMOLED”) displays include pixels each having its own pixel circuit. A very large number of pixel circuits have been proposed. A basic circuit design includes a two transistor, one capacitor (2T-1C) design. The transistors may be n-channel, p-channel, or a combination thereof. One transistor is a select transistor, and the other transistor is a driving transistor. Typically, the transistors are thin-film transistors (“TFTs”). TFTs and organic active layers degrade over time.
One pixel design that has been proposed to compensate for the degradation includes adding another transistor that is connected in series with the driving transistor. In some instances where n-channel transistors are used, the extra transistor would have its drain region connected to a Vdd power supply line and its source region connected to the drain region of the driving transistor. The source region of the driving transistor is connected to the anode of the OLED, and the cathode of the OLED is connected to a Vss power supply line. Even while on, the extra transistor adds resistance to the conduction path through the driving transistor and the OLED. The added resistance increases power consumption and generates more heat that needs to be dissipated without an increase in emission intensity of the OLED.
FIG. 1 includes an illustration of a pixel circuit, and FIG. 2 includes a corresponding timing diagram for the pixel circuit in FIG. 1. Referring to FIG. 1, the pixel circuit includes a select transistor 102, a capacitor 104, a driving transistor 106, and an OLED 108, which are configured similar to a 2T-1C pixel circuit. Node 105 lies between the select and driving transistors 102 and 106, respectively. The driving transistor 106 is a double-gated transistor, and a third transistor 122 has its drain connected to node 107. The voltages for Vdd, Vss, and signal line 162 are at substantially constant voltages. For example, Vdd can be approximately +13 V, Vss can be approximately −5 V, and signal line 162 can be approximately −12 V.
The timing diagram illustrates the voltages of selected parts of the circuit during one frame time. A first portion of the frame time is used to adjust voltages within the circuit, a second portion is used to write data into the pixel, and a third portion allows the pixel (i.e., OLED) to radiate.
During the first portion of the frame time, the top gate (“TG”) 166 of the driving transistor 106 is brought to a low-state (e.g., negative voltage), which turns off the driving transistor 106, i.e., no significant current is flowing from its drain to its source. The select line (“SL”) 142 is taken to an on-state and activates the select transistor 102 for the entire row of the display panel, and signal line (“G”) 164 is taken to an on-state and activates the third transistor 122. The voltages at nodes 105 and 107 are clamped to the voltage of the data line (“DL”) 144 and the signal line 162, respectively. The voltage across the capacitor 104 is thus the voltage difference between the DL 144 and the signal line 162. After a period of time, the top gate (“TG”) 166 is taken to a high state (e.g., zero or positive potential), and the signal G 164 is taken to an off-state state to deactivate the third transistor 122. The voltage (“Vc”) across the electrodes of the capacitor 104 decreases, and the voltage at node 107 increases and stabilizes at a voltage that is equal to the magnitude but opposite the polarity of the threshold voltage of the driving transistor 106. For example, the voltage of the node 107 is <2 V if the threshold voltage of the driving transistor 106 is 2 V.
During the second portion, data is written into the pixel. The signal on the DL 144 is transmitted to the node 105. The SL 142 receives an on pulse for a relatively short amount of time. TG 166 is taken to an off-state, and therefore, no significant current flows through the driving transistor 106, and the node 107 remains at its potential (e.g., less than 2 V). During the third portion, after which all data has been written for all rows (or columns, depending on the layout orientation), TG 166 is taken to a high state (zero or a positive potential) to activate the driving transistor 106 while transistor 102 is in an off-state to keep the node 105 isolated from the DL 144. The OLED 108 radiates due to a current defined by (Vnode 105-Vnode 107-Vth-OLED) during the writing period.
The pixel circuit 100 suffers from at least one or both of two problems, namely, low intensity and having to drive the electronics harder. A separate voltage adjust step is used. The adjust step biases portions of the pixel circuit 100 so that the OLED 108 no longer radiates until after all the data has been written to the array (end of the second portion). The time that the array radiates is less than half of the frame time. Although the various time periods are typically shorter than those of which a human can visually respond, a user may perceive the display as being dimmer, or the OLED 108 may need to be driven harder during radiation to make the display appear to have the proper emission intensity. Driving the OLED 108 harder increases the degradation rate of the driving transistor 106, the organic layer within the OLED 108, or both.